A high or medium voltage electrical cable essentially comprises a conductor, a conductor screen surrounding the conductor and an electrical insulation layer surrounding the conductor screen. High voltage electrical cables have robust dimensions with typical outer diameters between 70 to 120 mm. When there is a need to join two cables, one way is to prepare a flexible vulcanized joint between the cable ends. The conductor ends are normally jointed by welding, after which the insulation layer is restored by melting and curing an insulation material in a pressurized mould arranged around the cable. For enabling the welding of the conductors, the cable ends have to be stripped.
In order to improve the subsequent restoration of the insulation layer, it is a common practice to give the stripped cable ends a specific form comprising a conical insulation layer end. The tapering of the insulation layer is conventionally done with a tool known as a “pencil sharpener”. CN2596634 discloses an example of a tool of this type. The tool comprises two cutting edges arranged in the same plane as the axis of the cable, but inclined with respect to the axis of the cable. The cutting edges are turned about the cable axis. This tool achieves a sharp transition between the conductor screen and the insulation cone. The transition forms a cone edge that is conventionally shaped using a knife. In consequence, the cone edge often appears as a circular row of facets. The surfaces are finally given a finishing touch by grinding.
The geometry of the cone edge has a great significance to the cable performance as experience shows that most electrical breakdowns in flexible vulcanized joints at higher voltages occur at the cone edge. In presence of irregularities like facets following from shaping the cone edge with a knife, the breakdowns are initiated at the edges of the facets. Moreover, irregularities on the cone surface may lead to locally enhanced electric field strength and can also initiate an electrical breakdown. The described conventional method for stripping an electrical cable is unsatisfactory in that the quality of the resulting cable joint is strongly dependant on the skills of the person shaping the cone edge.
JP9311259 discloses a method and tool for cutting a cable terminal so that its surface slants at a specific angle relative to the axis of the cable terminal. The tool includes a housing provided with means for locking the cable to one end of the housing, and a rotating cutting blade which is brought into contact with an end of the cable during the cutting. The longitudinal axis of the cutting blade is arranged in the same plane as the axis of the cable, but inclined with respect to the axis of the cable. The cutting blade rotates about its axis and revolves around the peripheral surface of the cable to cut the surface of the cable in a tapered shape. The rotation of the blade around the peripheral surface of the cable is driven by means of a driving shaft and a handle.
JP06205520 discloses a device for stripping cables enabling formation of a tapered surface of a cable. The cable is held between a pair of holding bodies. The device comprises a cutter arranged for removing material from the cable, a movable guide frame moving the cutter in the direction of the radius of the cable at the same time as the cutter is moved along the cable axis, and rotational guiding means for guiding a rotation of the cutter about the longitudinal axis of the cable at the same time as the cutter is moved along the longitudinal axis of the cable. This stripping device achieves a helical cutting of the cone, and accordingly the surface of the cone obtains a screw pattern. Further, this device achieves a sharp transition between the cylindrical conductor screen and the cone shaped part of the insulation layer. In order to avoid electrical breakdown in the cable joints, the cable has to go through a machining step in which the cylindrical surface of the conductor screen, the cone edge, and the surface of the cone is grinded in order to achieve a soft transition between the cylindrical part and the conical part of the cable and a smooth surface of the cone as well as the conductor screen.
Thus, it is difficult to achieve a satisfactory cable joint quality with the currently available tools.